This invention relates to a circuit for providing balanced data transmission and, more particularly, to such a circuit for balancing such transmission over existing telephone wiring and for suppressing common mode currents in these lines.
The severity of the problems of transmitting high speed data over a regular twisted wire pair wire is legendary. These problems arise from a variety of conditions, including the capacitance of the wire (which increases with length), and the propensity of unshielded wire to pickup induced (capacitively or inductively coupled) voltages. In addition, electro-static discharge (ESD) voltages, which can be on the order of several kilovolts, applied to equipment by human operators, can cause transmission errors by impressing common mode voltages that exceed the equipment's limits on the wire pairs. Since many systems rely on a balanced pair of signals, any unbalance condition with respect to voltages on a conductor can have a severe impact. Over the years, many attempts have been made to balance such transmission lines using circuits which have come to be known as balun circuits; the word apparently coming from balanced-unbalanced. Many of these balun circuits have inductance coils which pass both signal wires through the coil windings, one signal wire per each winding. These windings are phased such that differential data signals pass unimpeded while common mode interference is rejected. Because of electro-static discharges, balun coils have, for many reasons, proven to be unsatisfactory.